1. Field of the Invention
The present invention relates to a honeycomb filter.
2. Discussion of the Background
Particulate matter (hereinafter, also referred to as “PM”) such as soot is contained in exhaust gases discharged from internal combustion engines such as diesel engines, and has raised problems as contaminants harmful to the environment and the human body. Also, people have been worried about influence of toxic gas components such as CO, HC, and NOx contained in exhaust gases on the environment and the human body as well.
For the above reasons, exhaust gas purifying apparatuses for collecting PM or converting toxic gas component in exhaust gases have been used.
Honeycomb structures made of ceramics or the like are used for producing the exhaust gas purifying apparatuses. When exhaust gases are passed through the honeycomb structure, those gases can be purified (converted).
A honeycomb structure for collecting PM in exhaust gases in an exhaust gas purifying apparatus has a large number of cells each sealed at either end thereof and placed longitudinally in parallel with one another with a cell wall interposed therebetween. Therefore, exhaust gases flowing into one of the cells surely pass through the cell wall separating the cells and then flow out from other cells. Therefore, when a honeycomb structure of this kind is installed in an exhaust gas purifying apparatus, PM contained in exhaust gases are captured by the cell walls upon passing through the honeycomb structure. The cell walls of the honeycomb structure function as filters through which the exhaust gases are purified.
WO2005/002709 A1 discloses a honeycomb structure in which cells each sealed at an end on the gas outlet side are defined as cells having a large volume (hereinafter, also referred to as large volume cells) and cells each sealed at an end on the gas inlet side are defined as cells having a small volume (hereinafter, also referred to as small volume cells).
In the honeycomb structure of this kind disclosed in WO2005/002709 A1, a total area of apertures on the gas inlet side is relatively made larger than a total area of apertures on the gas outlet side so that the honeycomb structure, when used as a filter for purifying exhaust gases, can capture a large amount of PM.
WO2005/002709 A1 also discloses a honeycomb structure characterized by that pores have an average pore diameter from 5 to 30 μm; a proportion of the volume of fine pores having a pore diameter of twice or more the average pore diameter relative to the total volume of the pores is 30% or less.
According to WO2005/002709 A1, a honeycomb structure having the above features has a large effective filter area, and the period to a regeneration process can be prolonged.
On the other hand, as a honeycomb structure to be used for converting NOx in exhaust gases in an exhaust gas purifying apparatus, a honeycomb structure for converting NOx in which no end of the cells is sealed and a catalyst for converting NOx is supported on cell walls has been known.
The contents of WO2005/002709 A1 are incorporated herein by reference in their entirety.